1. Field of the Invention
The present invention relates generally to x-ray systems, devices, and related components. More particularly, exemplary embodiments of the invention concern x-ray device components that include an emissive inorganic coating that can be applied with minimal surface preparation and that provides good resistance to corrosion and oxidation of substrates in high temperature environments. Depending upon the application, the emissivity of the coating employed in connection with a particular embodiment may vary.
2. Related Technology
Various aspects of the operation of x-ray devices often result in the exposure of many of the x-ray device components to extreme operating conditions that can damage or destroy those components over time. For example, the generation of x-rays, which generally involves accelerating electrons at high speed to a target surface on an anode, may result in operating temperatures as high as 1300° C. both at the anode and elsewhere within the x-ray device. The transmission of heat throughout the x-ray device is facilitated in large part by the conductive nature of the metallic components employed in a typical x-ray device. For example, the metal vacuum enclosure within which the cathode and anode are contained rapidly attains high operating temperatures due to exposure to the heat generated at the anode.
In addition to the aforementioned extreme thermal cycles, x-rays devices typically experience a variety of other unique operating conditions as well. For example, it was noted above that the anode and cathode are disposed in a vacuum enclosure. Generally, the vacuum enclosure is evacuated to a relatively high vacuum in order to ensure the removal of gases and other materials that may cause arcing due to the high potential difference between the cathode and the target surface of the anode.
The specialized operating environment wherein x-ray device components are required to function has stimulated the development of various approaches to the problems that frequently stem from sustained operation in such environments. Problems of particular concern are the degradation, and potential failure, of the metal x-ray device components that are exposed to extreme thermal cycles, vacuums, and other conditions.
Such degradation may be manifested, for example, in the form of corrosion and/or oxidation of metallic structures and surfaces. These effects are not limited to particular types of metal but, instead, generally appear without regard to the particular type of metal with which a component is constructed. For example, both corrosion and oxidation frequently appear in a variety of metallic components, regardless of whether those components are comprised of iron, steel, titanium, aluminum, or other metals.
Because problems such as corrosion and oxidation compromise the performance of the x-ray device and/or impair the integrity of x-ray device components, attempts have been made to prevent, or at least attenuate, these problems by way of various treatments of the metallic components of the x-ray device. Examples of such attempts include various surface treatment techniques, as well as the application of various types of coatings to selected metallic surfaces of the x-ray device components.
At least some of the attempts at coating the metal surfaces, for example, have been directed to improving the emissivity “∈” of the coated components so that, notwithstanding the extremely high operating temperature of the x-ray device, the emissive coating would nonetheless return a certain amount of heat back to the interior of the x-ray device, thereby reducing the temperature of the component or components to which the coating was applied. In other situations, it is desirable to provide a component with a coating of relatively low emissivity so that the coated component retains a significant portion of heat, and thereby substantially prevents the destructive transfer of heat to nearby systems and components.
As discussed in further detail below however, typical surface treatments, coatings, and associated processes are problematic and, in any event, often result in a component with emissivity that is either insufficiently low or insufficiently high, and that, accordingly, does little to enhance the overall durability or performance of the x-ray device.
For example, one surface treatment process often employed in connection with x-ray device components involves cleaning the stainless steel surface of a component using a grit blasting procedure. These types of procedures implicate significant problems however. In particular, grit blasting operations typically leave small grit particles embedded in the surface of the blasted part. While some types of embedded grit can be removed from the surface with some effort, it is difficult, if not impossible, to completely remove glass or alumina grit from the treated surface. This situation is of particular concern because the embedded grit may come loose from the surface during operation of the vacuum tube and cause arcing or other problems that can destroy the x-ray tube.
Another typical surface treatment process used in connection with x-ray device components involves firing the surface of the component in a wet hydrogen atmosphere at temperatures of about 900 degrees Celsius (“C”), or higher. However, while hydrogen firing desirably provides a green surface of somewhat improved emissivity, it is typically the case that grit blasting of the surface is required prior to greening in order to obtain more effective results. Such grit blasting of x-ray tube component surfaces can, as noted above, cause serious problems.
As suggested earlier herein, a related problem with both grit blasting and greening processes is that, notwithstanding the use of such treatments, the finished component surface nonetheless has a relatively low emissivity, typically in the range of about 0.2 to about 0.4. Among other things then, such surface preparation methods are ineffective in producing a coating or surface with an emissivity sufficiently high to be beneficial to the coated component. Moreover, even if the aforementioned emissivity level is acceptable, as in a case where the coated component is intended to retain a certain amount of heat, the grit blasting processes typically used in the attainment of that level of emissivity implicates serious problems, as suggested above.
The unique operational conditions that typify x-ray devices cause other problems as well with regard to typical x-ray device component surface treatments. For example, many x-ray device components comprise materials such as stainless steel that include some chromium. When the component, such as a vacuum enclosure, is greened in a wet hydrogen environment, oxidation of the surfaces of the component occurs and chromium oxide forms on those surfaces. However, the high vacuum inside the vacuum enclosure often causes the chromium oxide to separate from the inner surface of the vacuum enclosure during x-ray tube operations.
This is problematic at least because the separation of the chromium oxide causes the off-gassing of oxygen inside the vacuum enclosure. The presence of oxygen within the vacuum enclosure, in conjunction with the extremely high temperatures typically associated with x-ray tube operations, can result in combustion of some parts of the x-ray device and/or other destructive effects. Moreover, the presence of oxygen and chromium oxide within the vacuum enclosure may also contribute to arcing.
Similar problems occur when so-called ‘black iron’ coatings are used on x-ray device components. Generally, the application of black iron coatings involves plating iron on one or more surfaces of the x-ray tube component and then steaming the coated part at high temperature so that Fe3O4, or magnetic iron, is formed on the surfaces. Similar to the case of the chromium oxide coatings however, the vacuum inside the vacuum enclosure can cause separation of the magnetic iron from the surface of the coated component. The loose magnetic iron can cause arcing and other problems inside the vacuum enclosure.
A related problem with black iron coatings concerns the effects of the vacuum on the oxygen contained in the magnetic iron. In particular, the relatively high vacuum level often causes oxygen reduction, or dissociation from the magnetic iron. The off-gassing of oxygen in this way may cause serious problems with regard to the operation of the x-ray device, as discussed above. Moreover, the emissivity of the magnetic iron coating can be significantly impaired.
In view of the foregoing, it would be useful to provide x-ray tube components that include an emissive coating that is reliable, stable and effective in the extreme operating conditions typically associated with x-ray devices. In addition, the x-ray tube components should be such that the coating can be readily applied and effectively maintained with no or minimal surface preparation.